This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-160213, filed Jun. 7, 1999; and No. 2000-108099, filed Apr. 10, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a radiation detector using a thin-film transistor (TFT) as a read switching element.
A radiation detector that uses a TFT as a switching element for each pixel is formed by repeating processes for forming a thin film on one surface of a glass substrate, patterning the thin film by etching, forming another thin film to overlap the etched pattern, and patterning the latter thin film.
FIG. 1 shows the arrangement of a conventional radiation detector. The radiation detector has a plurality of pixels arrayed in a matrix. Each pixel 203 is constructed by a photoelectric conversion element 102 which can directly convert incoming radiation (e.g., X-rays) into a charge and is formed of selenium, a capacitor 103 for storing the generated charge, and a signal read TFT 101. A charge stored in the capacitor 103 of each pixel is read to an integrating circuit 112 including an amplifier 107, capacitor 110 and switching device 111, and multiplexer 109, via the TFT 101 and a signal line 105.
As shown in FIG. 2, a gate driver 108 turns on/off each TFT 101 by changing the potential on a vertical select line 106. Upon switching the TFT 101 between ON and OFF, an offset voltage xcex94Vout is generated by the TFT 101. The offset voltage xcex94Vout is given by:
xcex94Vout=(Cgs/Cout)xc3x97(Vonxe2x88x92Voff)
where Cgs is the effective capacitance 115 between the gate of the TFT 101 and the signal line 105, Von is the ON voltage of the TFT 101, Voff is the OFF voltage of the TFT 101, and Cout is the capacitance of a capacitor 110 of an integrating circuit 112, as shown in FIG. 3.
More specifically, when Cout is small, xcex94Vout increases. Especially, when the radiation detector is used in X-ray fluoroscopy, the integrating circuit 112 uses a capacitor 110 having a small capacitance Cout due to a weak detection signal, and the signal voltage must therefore be amplified. The offset voltage xcex94Vout is also amplified considerably together with the signal voltage. The amplified offset voltage xcex94Vout practically narrows down the dynamic range of the integrating circuit 112. Also, the amplified offset voltage xcex94Vout saturates the integrating circuit 112. These factors cause an S/N ratio drop.
It is an object of the present invention to improve the S/N ratio of a radiation detector.
The radiation detector comprises a plurality of charge conversion elements which are laid out in a matrix and convert incoming radiation into charges, a plurality of capacitors for storing the charges generated by the charge conversion elements, and charge read elements for reading the charges stored in the capacitors. Signals other than signals originated to charges stored in the capacitors, which are produced upon, e.g., turning on/off the charge read elements are canceled by adjustment means, thus improving the S/N ratio.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.